To meet the need of the majority, it is sought to develop surface coating, mastic and adhesive compositions that can be used or applied at low temperature (between 5° C. and 35° C.) and especially at room temperature (23° C.).
Many surface coating, mastic and adhesive compositions available on the market are manufactured from reactive polyurethanes based on MDI (methylene diphenyl diisocyanate), especially having isocyanate end groups that crosslink in the presence of moisture.
However, these compositions generally have the drawback of comprising sizeable residual contents of MDI originating from the polyurethane synthesis reaction, which are liable to lead to a certain number of drawbacks, especially toxicity problems.
NCO-terminated MDI-based polyurethanes are conventionally prepared by reacting a polyol with a stoichiometric excess of MDI. This stoichiometric excess may be expressed by an NCO/OH molar ratio (denoted “NCO/OH ratio”) of strictly greater than 1, which corresponds to the molar ratio of the number of isocyanate (NCO) groups to the number of hydroxyl (OH) groups borne by the reactive species bearing such functions, used in the preparation of the polyurethane. Said stoichiometric excess is necessary for obtaining isocyanate end groups on the polyurethane. Depending on the chosen NCO/OH molar ratio, the polyurethane synthesized is obtained with a greater or lesser excess of residual MDI monomer, corresponding to the MDI monomer that is unreacted at the end of the reaction.
In order to reduce the drawbacks associated with the presence of a high content of MDI, studies have been conducted to synthesize NCO-terminated MDI-based polyurethanes with a minimum amount of residual MDI monomer.
However, it has been observed that polyurethane compositions having an MDI monomer content of less than or equal to 1% by weight relative to the weight of the polyurethane composition are highly viscous at room temperature and have problems of stability over time in terms of viscosity.
Specifically, these polyurethane compositions with a low content of MDI monomer have a limited storage life and their viscosity rapidly increases over time, until they become too viscous to be able to be formulated in the form of a surface coating composition, an adhesive composition or a mastic composition, which can be used or applied at low temperature (5-35° C.) and especially at room temperature (23° C.).
It is known practice to use diluents such as hydrocarbon-based organic solvents or plasticizers to reduce the viscosity of NCO-terminated polyurethane compositions having a low content of residual diisocyanate monomer. However, these diluents have the drawback of having to be used in large amounts which is generally undesired. In addition, these diluents do not make it possible to efficiently stabilize the evolution of viscosity of such polyurethane compositions, especially in the long term.
Alternatively, it has been proposed in patent application WO 2011/051019 to add a C2-C24 monocarboxylic or dicarboxylic acid ester in order to reduce the viscosity of an NCO-terminated polyurethane composition having a low content of residual diisocyanate monomer. In the examples, it was demonstrated that by adding 5% to 6% by weight of a particular ester to an NCO-terminated polyurethane composition based on TDI (toluene diisocyanate) having a low content of residual TDI monomer, the viscosity of the composition no longer changed or changed very little over time after one week at 40° C.
However, the efficacy of this stabilization remains to be improved, especially in light of the amounts of stabilizer used and/or of the stabilization time.
Moreover, no stabilizing effect was demonstrated on NCO-terminated MDI-based polyurethanes having a low content of residual MDI monomer, since these compositions are generally more unstable in terms of viscosity than compositions based on NCO-terminated TDI-based polyurethane having a low content of residual TDI.
Thus, there is a need to provide an a polyurethane prepolymer that makes it possible to obtain a mastic composition having a low content of residual MDI-type monomer while maintaining a viscosity and mechanical properties that are satisfactory.